Zirconium phosphates include amorphous zirconium phosphates and crystalline zirconium phosphates including a 2-dimensional layered structure or a 3-dimensional network structure. Of these, hexagonal zirconium phosphates having a 3-dimensional network structure exhibit superior heat resistance, chemical resistance, radiation resistance and low thermal expansion properties and the like and are thus being researched for use as immobilizers for radioactive waste, solid electrolytes, gas adsorbing/separating agents, catalysts, antimicrobial agent raw materials, and fillers with low thermal expansion properties and the like. In particular, the silver-based inorganic antimicrobial agents obtained by supporting silver ions on hexagonal zirconium phosphates exhibit superior antimicrobial effects as well as excellent durability or long-lasting effects, are not readily discolored during resin processing and are superior in terms of product safety and may be thus utilized for processing into a variety of types of resins or applications.
Various hexagonal zirconium phosphates are currently known. For example, NaZr2(PO4)3, CaZr4(PO4)6, KZr2(PO4)3 and the like are disclosed in PTL 1 or NPL 1.
Known methods for synthesizing zirconium phosphates include a calcination method in which synthesis is carried out by mixing raw materials in a dry manner and then calcining the mixture at 1,000° C. or higher using a calcining furnace (for example, see PTL 2 and PTL 4), a hydrothermal method in which synthesis is carried out by mixing raw materials in water or mixing raw materials containing water and then heating under pressure, and a wet method in which synthesis is carried out by mixing raw materials in water and then heating at normal pressure (for example, see PTL 3), etc.
Among these synthesis methods, in the calcination method, it is not easy to uniformly mix raw materials and it is therefore difficult to obtain zirconium phosphate with a homogeneous composition with such a method. In addition, since crystallization by calcination causes the formation of agglomerated materials by calcination, it is necessary to grind these materials to convert the same into a powder with a specific particle size. However, since zirconium phosphates having high crystallinity have high hardness, abrasion of a grinding apparatus or contamination by abraded materials readily occurs. It is difficult to obtain particles having a particle size distribution with the narrow range disclosed in the present invention, since the shape or particle size of the ground crystals may not be controlled.
Meanwhile, a wet method or a hydrothermal method enables homogeneous zirconium phosphate salt powders to be easily obtained. However, the obtained powder is mainly a particulate crystalline powder with a size of 1 μm or less. To obtain a zirconium phosphate salt powder having a particle size higher than 1 μm at a high yield, it is necessary to age materials with a low concentration in an aqueous solution for a long period of time. For this reason, in practice, there are technical and economical difficulties. In addition, a method of screening and filtering out large particles from particulates obtained by a conventional wet method or hydrothermal method involves great costs and is not economical. That is, neither the preparation method of hexagonal zirconium phosphate particles having a particle size distribution with a narrow range, used for the present invention, nor the special characteristics thereof, are known.
Meanwhile, an antimicrobial agent in which antimicrobial metal ions are supported on these zirconium phosphate salts has been suggested. For example, PTL5 discloses the following Formula (1).M1aAbM2c(PO4)2.nH2O  (1)(In Formula (1), M1 represents a metal ion selected from silver, copper, zinc, tin, mercury, lead, iron, cobalt, nickel, manganese, arsenic, antimony, bismuth, barium, cadmium and chromium, A represents at least one ion selected from an alkali metal ion, an alkaline earth metal ion, an ammonium ion and a hydrogen ion, M2 represents a tetravalent metal, n represents a value satisfying 0≦n≦6, a and b are positive numbers, c and d satisfy c=2 and d=3, provided that la+mb=1, and c and d satisfy c=1, d=2, provided that la+mb=2, in which l is the valence of M1 and m is the valence of A.)
Ions such as silver, copper, zinc, tin, mercury, lead, iron, cobalt, nickel, manganese, arsenic, antimony, bismuth, barium, cadmium and chromium have been known for a long time as antimicrobial metal ions that exhibit antimold properties, antimicrobial properties, and antialgal properties. As antimicrobial agents possessing antimold properties, antimicrobial or antialgal properties, organic support antimicrobial agents in which antimicrobial metal ions are supported on an ion exchange resin, a chelate resin or the like, and inorganic antimicrobial agents in which antimicrobial metal ions are supported on a clay mineral, an inorganic ion exchanger or a porous body have been suggested. In particular, silver-based inorganic antimicrobial agents in which silver ions among antimicrobial metal ions are supported on an inorganic compound have properties of improved safety, long-lasting antimicrobial effects, and superior heat resistance as compared to a silver nitrate aqueous solution, thus having few restrictions on an application method, a storage method, a disposal method and applications as well as currently being applied to a variety of products. However, silver ions are unstable when exposed to heat and light and are immediately reduced into silver metals, thus having a problem of stability such as discoloration over a long period of time. Depending on the type of inorganic compounds to support the silver ions, the performance of the obtained silver-based inorganic antimicrobial agents varies and the antimicrobial agents frequently have restrictions.
Silver ions-supporting zirconium phosphate salts are known as materials which are chemically and physically stable and exert antimold properties and antimicrobial properties for a long period of time. Commonly, zirconium phosphate salts can be readily obtained as particulates and are thus easily applied to fibers, coatings and the like and are utilized in a variety of applications. However, in a case where a processed antimicrobial agent such as water processing materials is needed to be separated from water, or for a use in coatings or films with a specific thickness where crude particulate antimicrobial agents exhibit superior dispersibility, as compared to particulate antimicrobial agents and thus enable easy handling, conventional particulate zirconium phosphate salt powders have a disadvantage of poor separation from water, that is, a poor water permeation property, or poor dispersibility with respect to coatings or resins. Accordingly, there is a need for solutions to this disadvantage.